The performance of an aircraft is the product of its design capabilities in terms of aerodynamics, and environmental influences that affect these capabilities. The way in which an airplane performs in flight is important to know in advance, since the very standards in aviation related to take-off speeds, stall speeds, manageable load, and numerous other airplane maneuvers are dependent on certain criterion to ensure safe flight. This criteria is often referred to as performance limitations.
Establishing performance limitations is fundamental to the safe handling of an aircraft and is indirectly related to economic aspects of commercial aviation. When an engineer sits down to formulate an airplane design, the first item on his/her checklist is the aircraft performance.
Any modification to the aircraft or a new structure being added to the airplane which, in future, would compromise the aircraft performance, is simply crossed off. Atmospheric characteristics such as pressure, density, temperature, and humidity play a crucial role in altering the efficacy of the airplane engine and/or the aerodynamic capability of the aircraft to fly, thus affecting the aircraft performance.
The Correlation between Pressure, Density, and Temperature
To understand the relation between aircraft performance and the role of atmospheric factors, one must first understand the association of air pressure and air temperature with the density of air.
- With the increase of air pressure, a direct increase in atmospheric density occurs (given that the temperature is kept constant). The value of pressure in terms of International Standard Atmosphere (ISA) is “1013.25 hectopascals” and that of density is “1,225 grams per cubic meter”, as reported in Meteorology and Navigation. These values are taken as standard and were established at mean sea level. Similarly, a decrease in air pressure would result in a proportionate decrease in air density.
- If the pressure is kept constant, an increase in air temperature would result in decreased air density. Similarly, a decrease in temperature results in an increase in density.
In this video by Kwesi, the effect of pressurizing a parcel of air can clearly be observed. Notice how the air molecules come together at increased pressure, resulting in increased density (concentration). Conversely, when pressure is reduced the molecules move away from each other thus decreasing density.
Hence, the atmospheric pressure has a directly proportional relation to air density whereas; temperature is inversely proportional to the density of air. This connection significantly governs the aircraft performance.
Aircraft Performance and Air Density
Air density is vital to aircraft performance, mainly because of its role in the creation of lift and in maintaining a favorable fuel/air mixture. Variation in temperature and pressure alter the density of air and thus affect the above stated.
Aircraft Performance and Pressure Variation
Consider an aircraft taking-off from an aerodrome at sea-level and climbing to attain an altitude of 10,000ft. The standard ISA pressure lapse rate is “1hpa per 30ft”. If ISA conditions exist (pressure=1013.25hpa and temperature=15 degree Celsius) in that area, the pressure at 10,000ft would be 697hpa.
This decrease in pressure results in the decrease in density, which falls from 1225 gm/m-cube (ISA standard) to 905 gm/m-cube. Thus, reduced atmospheric pressure decreases aircraft performance in the following ways:
- Reduced atmospheric pressure alters the production of lift. Reduced density means lesser molecules of air flowing around the airplane’s wings to generate lift. The aircraft’s lifting capability is decreased.
- Reduced atmospheric pressure alters the fuel/air ratio. Reduced density, in this case, is responsible for less number of molecules of air entering the cylinder. This upsets the optimum ratio required between aircraft fuel and air molecules, to maintain efficient flight.
Aircraft Performance and Temperature Variation
Increase in temperature of a parcel of air creates hyperactivity amongst the air molecules; the molecules are energized and thus behave in a hyper manner to utilize this extra energy. The motion of the air molecules becomes highly erratic, and they collide with each other. These collisions expand the total volume of that specific parcel of air, thus decreasing density.
If an aircraft in flight enters from a parcel of air having a temperature of 20 degree Celsius, into a parcel of air having a temperature of 40 degree Celsius (while maintaining the same altitude), its performance would be significantly decreased. The increase in temperature gives way to decreased air density, which in turn affects the aircraft performance characteristics in terms of reduced lifting ability and reduced mass of air entering the cylinders for combustion.
To counter this reduced aircraft performance, devices such as superchargers and turbochargers are used to condense the atmospheric air to its optimum density, required by the engine.
Meteorology and Navigation, Aviation Theory Centre, 1999.
Aeroplane General Knowledge and Aerodynamics, Aviation Theory Centre, 2004.
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